Known power storage devices include, for example, secondary batteries, such as lithium ion batteries, nickel hydrogen batteries, and lead batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to downsizing of cellular phones or restriction in installation spaces, and the like, power storage devices are further sought to be downsized, and thus attention is being drawn to lithium ion batteries having high energy density. Metal cans that have been used for packaging materials for lithium ion batteries are being replaced by multilayer films due to their light weight, high heat dissipation, and low manufacturing cost.
Such a lithium ion battery using the multilayer film as a packaging material uses a configuration in which battery contents (e.g., positive electrode, separator, negative electrode, electrolyte solution) are covered with a packaging material including an aluminum foil layer to thereby prevent moisture from penetrating into the battery. A lithium ion battery using such a configuration is referred to as an aluminum laminated lithium ion battery.
Examples of such an aluminum laminated lithium ion battery include embossed lithium ion batteries which are well known. In such an embossed lithium ion battery, for example, a recess is formed in part of a packaging material by cold forming to accommodate battery contents therein, and the remaining part of the packaging material is folded, followed by sealing the edge portion by heat sealing (hereinafter may be referred to as “single-sided battery”). In these years, lithium ion batteries have also come to be manufactured for the purpose of enhancing energy density. Such lithium ion batteries are produced by forming a recess on both sides of a packaging material whose surfaces are bonded together to accommodate more battery contents (hereinafter may also referred to as “double-sided batteries”). The double-sided batteries have a problem of difficulty in aligning when bonding the surfaces of the packaging material to each other. However, for a single-sided battery to obtain the energy density equivalent to that of a double-sided battery, a deeper recess is required to be formed.
The energy density of the lithium ion battery can be made higher with a deeper recess formed by cold forming. However, when a deeper recess is formed, pinholes or breakage are more easily caused in forming the packaging material, and thus formability is deteriorated. Therefore, a polyamide film having good formability is used for the base material layer of the packaging material, and the base material layer is bonded to a metal foil via an adhesive layer to protect the metal foil. In this case, the base material layer is required to be in intimate contact with the adhesive layer. As a method of improving the intimate contact of the base material layer with the adhesive layer, there is known a method in which a surface of a base material layer is corona-treated (e.g. see PTLs 1 and 2).
A polyamide film has poor resistance to an electrolyte solution that is a content of the lithium ion battery. When an electrolyte solution unavoidably adheres to the polyamide film in injecting the electrolyte solution during production of the lithium ion battery, the polyamide film will melt and poor appearance may be created. Thus, in a proposed packaging material imparted with electrolyte resistance on a surface of the base material layer, a polyethylene terephthalate (PET) film is further laminated on the outer side of a polyamide film (e.g. see PTL 3).